Junqiang Li

Spatial multiuser access with MIMO smart antennas for OFDM systems

Indoor wireless communication systems have grown rapidly because of their clear advantages such as mobility, flexibility, and inexpensive network reconfiguration. In order to offer higher data rates approaching those provided by wired LANs, a combined OFDM/SDMA-based approach is an effective solution for increasing the system capacity and spectral efficiency. However, in multi-user environments, the system performance is limited by co-channel interference. Multi-input-multi-output (MIMO) smart antennas with prior knowledge of the channel at the transmitter is another promising technique for providing significant increase in system capacity and performance in wireless communication systems. We investigate the use of smart antennas at both the base and mobile stations, operating jointly, to maximize the SINR of each user before multiuser detection. By doing so, the performance of multi-user detection is significantly improved.

Adaptive cochannel interference cancellation in space-time coded communication systems

Space-time coding is a powerful scheme that combines channel coding, modulation, and multiple transmit antennas to achieve higher data rates and combat fading in wireless systems. In this letter, we propose a multiple-input multiple-output minimum mean-square error spatial-filtering-based adaptive antenna arrays method to suppress cochannel interference (CCI) in space-time coded systems. It is shown that the proposed method can effectively suppress CCI while preserving the space-time structure, thereby significantly improving the systems interference suppression ability without significant bit error rate performance degradation.

A reduced-complexity maximum-likelihood method for multiuser detection

Multiuser or joint detection has recently been receiving significant research interest because of its potential for the significant increase in system capacity and performance. Among the conventional multiuser detection schemes such as the minimum mean-squared error and successive interference cancellation-based ones, maximum-likelihood multiuser detection has the best system performance. However, the complexity of maximum-likelihood detection (MLD) increases exponentially in the number of users and constellation size. In this paper, a low-complexity MLD scheme based on the use of a sensitive-bits algorithm is proposed. It is demonstrated that the proposed method can greatly reduce the computational complexity with a minimal penalty in performance compared with the exhaustive optimal MLD scheme.

Multi-stage low complexity maximum likelihood detection for OFDM/SDMA wireless LANs

Space division multiple access (SDMA) is a promising solution for the significant increase of system capacity and spectral efficiency in wireless LANs. To make the system more robust against the detrimental effects of frequency selective fading channels, a combined OFDM/SDMA based WLAN has been previously proposed. In particular, it has been shown that MLD (maximum likelihood detection) has the best SNR performance compared with other detection schemes such as LMMSE and SIC (successive interference cancellation) based ones. However, the complexity of MLD increases exponentially in the number of users and constellation size. In this paper, a new multi-stage low complexity MLD scheme using the sensitive bits algorithm (SB-MLD) is proposed. It is shown that it can greatly reduce the computational complexity with a minimal penalty in performance compared with the exhaustive MLD scheme.

Reduced-complexity MAP-based iterative multiuser detection for coded multicarrier CDMA systems

In recent years, combining multiuser detection (MUD) and channel decoding has received considerable attention. The maximum a posteriori (MAP) criterion-based iterative multiuser detector greatly improves the system performance and can approach the performance of single-user coded systems. However, its complexity increases exponentially with the number of users and can become prohibitive for systems with a medium-to-large number of users. In this paper, a reduced complexity MAP-based iterative MUD based on the use of a soft sensitive bits algorithm is proposed for coded multicarrier code-division multiple-access systems. It is shown that it can greatly reduce the computational complexity with a minimal penalty in performance compared to the conventional optimal scheme.

Space-time turbo multiuser detection for coded MC-CDMA

The system capacity and performance of multicarrier code-division multiple-access (MC-CDMA) communication systems can be significantly enhanced by jointly employing MAP-based multiuser detection (MUD) and channel decoding techniques. In this paper, a group-oriented soft iterative MUD based on the combination of smart antennas and iterative MAP-based MUD is presented. The proposed method is featured as a novel technique for further increasing the system capacity and performance. In this method, all the users are first grouped into several groups according to their impinging direction of arrivals (DOAs). All users with similar DOAs are classified into the same group and then low-complexity MAP-based iterative MUD is employed in each group. Because spatial filtering cannot suppress all the interference between the groups, interference cancellation among the groups is used prior to MUD within each group. It is shown that the proposed group-oriented soft iterative MUD algorithm can significantly reduce the computational complexity compared with the conventional optimal MAP-based MUD schemes. It is also demonstrated that the performance of the proposed algorithm can approach that of a single-user coded MC-CDMA system with an antenna array in additive white Gaussian noise and frequency selective fading channels.

A group oriented multiuser detection with beamforming for multicarrier CDMA systems

The capacity of MC-CDMA communication systems can be significantly enhanced by employing multiuser detection (MUD) techniques. A group oriented MUD based on the combination of smart antennas and multiuser or joint detection is presented. The proposed method is featured as a novel technique for further increasing the system capacity and performance. In this method, all the users are first divided into several groups according to their impinging direction of arrivals (DOA). All users with similar DOAs are classified into the same group and then MUD is employed in each group in parallel. Because spatial filtering cannot suppress all the interference between the groups, interference cancellation among groups is used prior to MUD within the groups. It is shown that the proposed group oriented MUD algorithm can significantly reduce the computational complexity and processing time delay compared with the conventional MUD schemes. Furthermore, it is demonstrated that significant performance improvement can be achieved.

Co-channel interference cancellation for space-time coded OFDM systems

Space-time coded OFDM is a promising scheme for future wideband multimedia wireless communication systems. The combination of space-time coding (STC) and OFDM modulation promises an enhanced performance in terms of power and spectral efficiency. However, STC transmit diversity impairs the systems interference suppression ability because each transmitter generates multiple signals that appear independent to the receiver antenna arrays. In this paper, we propose a novel CCI cancellation method employing angle diversity based on beamforming. It is shown that the proposed method can effectively mitigate CCI while preserving the space-time structure, thereby, significantly improving the systems interference suppression ability without significant performance degradation.

Reduced complexity MAP-based iterative multiuser detection for coded multi-carrier CDMA systems

In previous years, combining multiuser detection (MUD) and channel decoding has received considerable attention. The maximum a posteriori (MAP) criterion based iterative multiuser detector greatly improves the system performance and can approach the performance of single user coded systems even for moderate signal-to-noise ratios. However, its complexity increases exponentially with the number of users, and can become prohibitive for systems with medium to large number of users. A reduced complexity MAP-based iterative MUD based on the use of a soft sensitive bits algorithm is proposed for multi-carrier CDMA systems. It is shown that it can greatly reduce the computational complexity with a minimal penalty in performance compared to the conventional optimal scheme.

A group oriented soft iterative multiuser detection for coded multi-carrier CDMA systems

The system capacity and performance of MC-CDMA communication systems can be significantly enhanced by jointly employing MAP-based multiuser detection (MUD) and channel decoding techniques. In this paper, a group oriented soft iterative MUD based on the combination of smart antennas and iterative MAP-based MUD is presented. The proposed method is featured as a novel technique for further increasing the system capacity and performance. It is demonstrated that the performance of the proposed algorithm with low complexity can approach that of a signal user coded MC-CDMA system with an antenna array in AWGN and frequency selective fading channels.